Method and apparatus for robot-supported placement of objects

ABSTRACT

An apparatus includes a manipulator, a pneumatic actuator and a robot control. The pneumatic actuator is coupled to the manipulator such that the pneumatic actuator is arranged between the manipulator and an object to be positioned. The robot control is configured: to control the pneumatic actuator such that an actuator force of the pneumatic actuator approximately compensates a weight force of the object; and to monitor the actuator position of the pneumatic actuator and initiate safety measures when the actuator position exceeds a predefined value. Methods of operating the manipulator and the pneumatic actuator are also described.

TECHNICAL FIELD

The invention relates to a robot-assisted method for positioning orstacking objects, for example for stacking workpieces by means of amanipulator.

BACKGROUND

The problem frequently arises in automated production of stacking aplurality of uniform or similar objects. For example, completed,optionally already packaged, objects are stacked on pallets for thepurpose of transport or semi-finished workpieces are stacked at a(temporary) storage position to process them further at a later date. Asimilar task is the loading and unloading of boxes and crates withobjects for the purpose of transport. Industrial robots having aplurality of degrees of freedoms (manipulators) and which are able toposition objects practically as desired are used for this purpose inmass production.

A simple robot suitable for such purposes is described in thepublication DE 40 18 684 C2. A pivotable arm, which is further dividedand is variable in its length, is attached to a vertical column. Aseparate gripping apparatus for the object to be manipulated is attachedto the end of the arm.

The robot control of an industrial robot is typically programmed so thatthe so-called “tool center point” (TCP) runs along predefinedtrajectories, planned in advance, whereby there are various methods forplanning such motion paths. The planned trajectories and the taskscarried out along the paths (e.g. “pick tool” “release tool”) aretogether referred to as a robot program. The robot program is normallyadapted to the objects to be manipulated. The robot control thus needsto “know”, e.g. when stacking objects, how high the object is in orderto place the next object on the previous one. Assuming the first object(e.g. a box in which a product, for example a fragile object or afoodstuff is packaged) is placed on the ground, with the object (thebox) being 20 cm high. The robot then has to position the next objectsuch that its bottom side rests at a height of 20 cm above the groundbefore the object is released. The next object after that is thenpositioned 40 cm above the ground to place it on the previous one, andso on. In a similar manner, the robot needs to “know” the shape and theposition of a housing or of a packaging when placing an object into thehousing or into the packaging. Robots intended for such purposes arealso known as “palletizing robots”.

The description above outlines an ideal case in which the dimensions ofthe object to be positioned and/or stacked, and optionally thedimensions of the housing, are known and are strictly adhered to. Ife.g. the actual height of an object differs from a specified dimension,when the manipulator places down the next object it may collide with theprevious object (if its height is larger than the specified height) orthe object may be released at too early a time and fall onto theprevious one (if its height is smaller than the specified height). Thetolerances of the individual objects add up during stacking. To avoidcollisions, the robot program can be configured such that an object isalways released at a specific distance above the level at which itshould be placed down in the ideal case. In this case, the object willalways fall a short distance, however. Dropping the objects over a shortdistance (some few mm) may not be a problem in many applications;however, there are some applications in which the objects to bemanipulated have to be handled with care and a “gentle” manipulation ofthe objects is necessary. Such a “gentle” manipulation is not possiblewith conventional industrial robots (without complex and/or expensiveadditional sensor systems) because the robot control controls themanipulator such that the TCP runs along the (e.g. point-by-pointdefined) motion path to an exact end point (a specified position); therobot therefore always moves to (previously) defined deterministicpositions and cannot take account of (random) deviations and tolerancesin the objects to be manipulated.

SUMMARY

The problem underlying the present invention is therefore to find amethod which allows a “gentle” positioning or stacking of objects. Themethod should be robust with respect to variations in the actualdimensions and the destination positions of the objects. The objectsshould in particular be placed down on contact and should never bedropped. This problem is solved by the method and apparatus embodimentsdescribed herein.

A method for positioning, in particular palletizing, objects isdescribed. The method is carried out by means of a manipulator having anadditional actuator which is arranged between the manipulator and theobject to be positioned. In accordance with one example of theinvention, the method comprises the picking up of the object and themoving of the object, by means of the manipulator, to a start positionclose to a placement surface on which the object is to be positioned andplaced down. The method further comprises the moving of the object bymeans of the manipulator toward the placement surface, with the actuatorbeing controlled such that the actuator force compensates the weight ofthe object or with the actuator force being regulated such that anadjustable minimal net actuator force acts on an end stop of theactuator (and can be zero in a borderline case). The deflection of theactuator is furthermore monitored and a change of the deflection isdetected. The movement of the manipulator is stopped upon detection of achange of the deflection of the actuator, whereupon the object can bereleased.

In accordance with a further example of the invention, the methodcomprises the controlling of the actuator such that the actuator forcein the vertical direction corresponds to the weight force of the object;and it comprises the initiation of safety measures if the deflection ofthe actuator has changed by at least a predefinable value.

Further, an apparatus for positioning objects is described. Inaccordance with a further example of the invention, the apparatuscomprises a manipulator having a robot arm movable in a horizontal planeand having a lifting apparatus which is configured to position the robotarm in the vertical direction. The apparatus further comprises anactuator which is arranged between the workpiece and the object to bepositioned and a robot control which is configured to control theactuator such that the actuator force in the vertical directionapproximately compensates the weight force of the object. The robotcontrol is further configured to monitor the deflection of the actuatorand to initiate safety measures as soon as this deflection exceeds apredefinable value.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference toFigures. The Figures and the further description should help to betterunderstand the invention. The details shown here are not necessarily tobe understood as a restriction; instead importance is placed onexplaining the principle underlying the invention. The same referencenumerals designate the same or similar components having the same orsimilar significance. There are shown in the Figures:

FIG. 1 an example of an industrial robot which is suitable for stackingor positioning objects;

FIG. 2 an actuator which is arranged at the robot, is mechanicallydisposed between the robot and the object to be manipulated, and whichallows a relative movement almost free of static friction between therobot and the object;

FIG. 3 schematically, a first part of a palletizing process;

FIG. 4 schematically, a second part of a palletizing process;

FIG. 5 a schematic representation of the pressure regulation for theweight force compensation by the actuator; and

FIG. 6 a schematic representation of possible emergency-stop scenariosin accordance with the described embodiments.

DETAILED DESCRIPTION

An example of a manipulator (robot) is shown in FIG. 1 which is suitablefor coping with the previously mentioned stacking and positioning work.The manipulator 10 comprises a robot arm 12 which can be moved up anddown in the vertical direction along a fixed column 11 by a liftingapparatus. In the present example, the column is firmly connected to aholder 15 which can, in turn, be firmly anchored on the floor or to afoundation. In the present example, the robot arm 12 comprises twojoints 13 and 14, with the robot arm being connected to the liftingapparatus in an articulated manner via the joint 14. The joint 13connects two segments of the robot arm (upper arm or proximal segment,lower arm or distal segment). The joints 13 and 14 are designed suchthat the arm can carry out a movement in a horizontal plane. Thevertical position of this horizontal plane can be fixed by the liftingapparatus. An actuator 20 is mounted on the end of the robot armopposite the joint 14 and can—to a small extent—carry out a verticalmovement z_(REL) relative to the robot arm. The actuator 20 canoptionally also be rotatably connected to the robot arm 12 in anarticulated manner such that the actuator 20 is rotatable about its own(longitudinal) axis (pivot joint 16). The actuator 20 connects a tool(e.g. a gripper) to the robot arm, with the vertical position z_(REL) ofthe tool relative to the robot arm (i.e. deflection of the actuator oractuator position) being adjustable by means of the actuator 20.

The actuator 20 is, in particular, free of static friction, whereas freeof static friction in, in this connection, understood to mean that thatthe static friction force acting between the movable parts of theactuator is negligibly small, in particular smaller than 5% of theeffective actuator force. The actuator force is the force exerted by theactuator 20 on the tool (e.g. the gripper).

The actuator 20 is a linear actuator which can e.g. be configured as anelectrical direct drive or also as a pneumatic actuator. With pneumaticactuators, the use of actuators free of (static) friction or almost freeof friction such as bellows cylinders or air muscles can be consideredwhich e.g. work against a spring element which effects a restoringforce. The actuator 20 can also have a dual-action pneumatic cylinder.In this case, no spring is necessary for exerting a restoring force. Thestatic friction acting between the movable parts of the actuators isproblematic in positioning work—and in particular in the stacking ofobjects. A bellows cylinder or an air muscle itself does not have anystatic friction since no parts moving with respect to each other have toslide on one another. A negligible static friction in the cylinder canalso occur with pneumatic cylinders. That is, with a maximum actuatorforce of 200 Newtons, the force required to overcome the static friction(e.g. at a static friction of 1% of the maximum force) amounts to just 2Newtons. Actuators with such a small static friction are calledactuators “free of static friction”. Conventional actuators exhibitabout 20 times more static friction. In order to also avoid(significant) static friction in the bearings, the actuator can e.g.have recirculating ball bearings. The practical freedom of the actuatorfrom static friction is of great advantage for a precise forceregulation. A friction force higher by one Newton would, in the aboveexample, result in an approximately 10 Newton greater inaccuracy in thedynamic force regulation (i.e. at the varying positions of theactuator). An exact force regulation is required to allow a placing downof the object to be positioned (or to be stacked) which is as gentle aspossible and to avoid shocks to the greatest possible extent.

FIG. 2 shows a sectional representation of an exemplary embodiment ofthe actuator 20 of FIG. 1. In the present case, the actual adjustmentelement is a compressed air-driven bellows cylinder 114 which worksagainst a spring 115. The actuator 20 comprises a first flange part 110as an interface to the industrial robot (manipulator) and a secondflange part 111 at which a gripper can, for example, be arranged. Ahousing part 112 is rigidly connected to the first flange part 111 andboth a regulation valve 113 and a static friction free shaft guide(shaft 116, (recirculating ball) shaft guide 117) with the restoringspring 115 are arranged in said housing part. A bellows 118 is providedas a cover between the flange parts 110 and 111 for protection againstdust and other contaminants. This cover can also be configured asliquid-tight and/or dust-tight for use in a rough environment. Thebellows cylinder 114 serves as a gearless adjustment element free ofstatic friction. The adjustment element acts between the first housingpart 112 and a second housing part 112′ rigidly connected to the holder111. To allow force regulation, the actuator comprises a path sensor(not shown or covered by the shaft guide 117) and a pressure sensor. Theactuator force can be calculated by measuring the pressure in thebellows cylinder and by the position of the bellows cylinder (by meansof the path sensor). Since the kinematics of the robot are known, theforce exerted on the object to be manipulated can also be determined(and consequently also regulated) via the measured parameters.Connections for the air inflow and outflow ducts 119 can e.g. bearranged in the first housing part 112. The connection to the air inflowduct is e.g. connected to a compressor via a hose.

Regardless of its actual implementation, the actuator 20 comprises twoend stops. The first end stop defines the minimal deflection (z_(REL)=0)of the actuator 20 and the second end stop defines the maximumdeflection (z_(REL)=z_(REL,max)) of the actuator 20. The maximumdeflection z_(REL,max) can, for example, be 100 mm and typically liesbetween 50 mm and 250 mm.

Reference is made to FIGS. 3 and 4 in the following. The workpiece(object 50) to be positioned is first moved by means of the manipulator(e.g. the manipulator 10 in accordance with FIG. 1) to a start positionz_(START) for a positioning task, i.e. to position or palletize (stack)objects. This start position z_(TART) is located in the vicinity of thedesired placement position z_(A) at which the workpiece 50 should cometo rest at the end. In a palletizing process, this start positionz_(START) is above the desired placement position z_(A) in a verticaldirection, with the vertical distance (z_(START)-z_(A)) between thestart position z_(START) and the placement position z_(A) essentiallybeing a safety distance which has to be observed in order not to riskcolliding with the already stacked objects. The actuator 20 which isarranged between the workpiece 50 and the manipulator 10 is located inits end position with a maximum deflection z_(REL,max) in this phase ofthe positioning process, with the actuator 20 pressing against the endstop with an (adjustable) minimal force F_(MIN). This (negative) minimalforce F_(MIN) is selected in this respect to be as small as possible,typically in ranges from −10 to −1 Newton. The weight force F_(G) of theobject in this respect has to be almost completely (except for the namedminimal force F_(MIN)) compensated by the actuator. This situation isshown in FIG. 3. The actuator pulls the workpiece 50 upward with a(negative) force F_(X), while the (positive) weight force of the object50 acts downwardly. Here F_(X)=−(F_(G)+F_(MIN)), i.e. the net forceacting on the end stop of the actuator 20 is the above-mentioned minimalforce F_(MIN)−(F_(G)+F_(X)). The balance between the actuator forceF_(X) and the weight force F_(G) can also be disturbed by forces arisingfrom the acceleration of the manipulator 10. This disturbing force can,optionally, be measured or calculated and taken into accountaccordingly. The corresponding information (e.g. position data and theirfirst and second derivations) are known to the robot control of themanipulator.

Once the workpiece 50 has been brought into the start positionz_(START), the placement process can begin. For this purpose, themanipulator 10 is controlled such that the actuator 20 together with theworkpiece is moved from the start position z_(START) toward theplacement position z_(A), with the actuator 20 still (only) pressingagainst the end stop with the minimal net force F_(MIN) and having fulldeflection z_(REL) max. The speed v is adjustable and at a constantspeed the current position z(t) of the workpiece 50 is equal toz_(START)−v*t (where the speed v is a positive value andz_((t=0))=z_(START)).

At the time to at which the workpiece reaches the placement positionz_(A), the workpiece 50 contacts the workpiece 51 (or generally thedesired placement surface) disposed thereunder. Due to the movement ofthe manipulator 10 (and thus of the actuator 20 and of the workpiece50), the actuator 20 is compressed directly after contact between theworkpiece 50 and the placement surface at position z_(A) and the netactuator force F_(MIN) no longer acts on the end stop, but on theplacement surface (e.g. the upper side of the workpiece 51). Thedeflection z_(REL) of the actuator 20 simultaneously decreases(dz_(REL)/dt<0). The robot control does not have to know the actualplacement position z_(A) nor does the placement position z_(A) have tobe measured. A change of the deflection can be evaluated for the contactrecognition. For example, an evaluation of the inequationdz_(REL)/dt<=or z_(REL)<z_(REL,max) can take place in order to recognizethe contact between the workpiece 50 and the placement surface. Sinceboth the position of the manipulator 10 and the deflection z_(REL) areknown at this time, this information can be used to gauge unknownobjects or to recognize their dimensions (e.g. the vertical coordinateof the contacted surface). A “resilience” or “stiffness” of thecontacted object can be recognized from the combination of differentminimal forces F_(MIN) and the arising deflection z_(REL) after contact.This e.g. allows the recognition of an unstable (and thereforeresilient) placement surface, for example, if, in a stack of boxes, thelower box on which another is to be placed is damaged. The determinationof the resilience can take place upon contact between the object to bepositioned and the placement surface (that is, upon placing down) orupon contact between the object to be picked up and the actuator (thatis, upon gripping).

As soon as the contact (i.e. the placing down of the workpiece 50) hasbeen recognized, the movement of the manipulator 10 is stopped, wherebythe speed v need not be set to zero abruptly, but may instead be slowlyreduced to zero in order to reduce the dynamic forces. The timeavailable for braking the manipulator 10 is essentially determined bythe maximum stroke z_(REL, max)−z_(REL,min) of the actuator 20. Theforce regulation of the minimal net actuator force F_(MIN) is activeduring the entire placement process. This situation is shown in FIG. 4.It is ensured in this manner that, in the phase from the first contactbetween the actuator 20 and the workpiece up to the stopping of themanipulator 10, a force is never exerted on the workpiece by themanipulator which is higher than the (adjustable) minimum force F_(MIN).

At the end of the placement process, as soon as the manipulator 10 isstationary and the deflection of the actuator 20 is constant again, theweight of the workpiece 50 is transferred to the placement surface. Thismeans that the net actuator force is increased until the actuator forceF_(X) is zero and the net actuator force is equal to the weight forceF_(G). The force regulation can then (while maintaining the currentdeflection z_(REL) of the actuator 20) be switched off and the workpiececan be released. Subsequently, the actuator 20 is moved by a furtherreduction of the deflection z_(REL) and/or by a movement of themanipulator away from the placed workpiece and the next workpiece can bepicked up.

The described process offers the advantage that only the start positionx_(START) of the robot control (of the manipulator 10) has to be known.This can be easily calculated if, for example, the maximum dimension(height) of the workpiece 50 is known as well as the number ofworkpieces already placed down plus a possible safety distance. Acollision is thus prevented even in the case of comparatively largetolerances of the dimensions of the workpieces. The actual placementposition z_(A) (e.g. the current height of the stack) does not have tobe known to the robot control and therefore also does not have to bemeasured. The detection of the actual placement position takes place bymeans of the actuator 20 whose deflection z_(REL) is monitored, wherebythe net actuator force F_(MIN) is regulated to a value close to zerobefore placing down the workpiece at its placement position z_(A) tokeep the impact forces upon contact as small as possible. It can beimportant in this connection that the actuator 20 is practically free ofstatic friction, that is no significant slip-stick effect occurs duringa change of the actuator position z_(REL) (e.g. upon compression of theactuator).

Even if the contact forces are kept very small by means of theabove-described actuator (actuator 20, FIGS. 1 to 4), it may be possiblewith some applications to provide safety measures which e.g. prevent anunwanted squeezing of objects or persons if an unplanned collisionoccurs between a workpiece manipulated by the robot (workpiece 50, FIGS.3 to 6) and another object or a body part of a person. The dimensions ofan object can also lie outside a certain tolerance. In addition toavoiding damage, the higher ranking control can also then also receive amessage (signal) about this.

Three different states of the actuator burdened with a workpiece 50(weight force F_(G)) are shown in FIG. 5. The actuator 20 (see FIGS. 1to 4) is represented in this illustration by a pneumatic cylinder 114′which can be used alternatively to the bellows 114 (see FIG. 2). Thecylinder is operated such that the workpiece 50 is raised when thecylinder pressure p_(Z) exceeds a minimum value p_(M). The examplemarked by “Case 1” in FIG. 5 shows the case of a pressure drop in thecylinder 114′ of the actuator 20 (actuator force F_(X)=0). As aconsequence of this, the weight (weight force F_(G)) of the workpiece 50pulls the actuator into the lower end position, marked by z_(A) in FIG.5. The example marked by “Case 2” in FIG. 5 shows the case of an “excesspressure” (p_(Z)>p_(M)) in the cylinder 114′ of the actuator 20(actuator force F_(X)>F_(G)). As a consequence of this, the actuatorpulls the workpiece 50 (weight force F_(G)) up to the lower end positionof the actuator 20, marked by z_(B) in FIG. 5. The example marked by“Case 3” in FIG. 5 shows the (normal) case of weight compensation. Thismeans that the cylinder pressure p_(Z) is regulated to that pressurep_(M) which is required as a minimum to compensate the weight forceF_(G) of the workpiece (actuator force F_(X) is equal to the weightforce F_(G)). The workpiece hovers, so to speak, at a (predefined)specified position z_(C). It must be noted in this connection that thepositions z_(A), z_(B), z_(C) and z_(D) shown in FIG. 5 do not representany absolute positions of the workpiece 50, but rather deflections ofthe actuator 20. The absolute position of the workpiece 50 is determinedby the position of the manipulator 10 (see FIG. 1) on which thedeflection of the actuator 20 is superposed. In the “case 3” shown inFIG. 5, the specified position z_(C) is therefore to be understood as arelative position with respect to the position predefined by themanipulator (cf. FIG. 1).

Two different safety measures are shown in FIG. 6 which can be carriedout in different situations in order, for example, to prevent(excessive) squeezing of a person or of an item. In the illustration onthe left in FIG. 6, the normal case of weight compensation is shownagain. The workpiece 50 is positioned by means of the manipulator 10(see FIG. 1) and the actuator 20 provides a “hovering” of the workpiece50 on an actuator deflection z_(C) (actuator force F_(X) equal to weightforce F_(G), no static friction). If the workpiece 50 collides with anobstacle upon lowering of the workpiece 50 (for example as part of astacking process or of a positioning task as described further above), acollision force F_(K) acts on the workpiece 50 which “compresses” theactuator 20; the deflection z of the actuator 20 therefore becomessmaller (z<z_(C)). The workpiece 50 is raised relative to the actuator20; the movement of the workpiece 50 is blocked by the obstacle whilethe manipulator 10 continues to move. As soon as the actuator deflectionfalls below a minimum deflection z_(D) (z<z_(D)), a safety measure iscarried out, for example an emergency stop.

In accordance with the examples of the invention described here, atleast two measures can be distinguished, namely an emergency stop of themanipulator 10 and of the actuator 20 during a slow vertical movement ofthe manipulator 10 (emergency stop type A) and during a fast verticalmovement of the manipulator 10 (emergency stop type B). In both cases,the power supply for the motors of the manipulator 10 is interrupted andthe brakes (where present) of the lifting apparatus (cf. FIG. 1 column11) engage immediately. In the case of a slow manipulator movement, thebraking distance is short, for example 1 mm, whereas the brakingdistance can be significantly longer (e.g. 9 mm) in the case of a fast(maximum) manipulator movement. The actuator 20 also has to absorb thisbraking path, whereby the former is further compressed. The differencebetween a “critical deflection” z_(D) of the actuator, upon which thesafety measure (emergency stop type B) is carried out and the minimumdeflection z_(B) of the actuator therefore has to be (configured) largerthan the maximum braking distance.

Further, in the case of only a slow vertical movement of the manipulator10 (emergency stop type A), the valves of the pneumatic adjustmentelement in the actuator 20 (in the present case the valves of thecylinder 114′) are blocked and the workpiece is only lowered slowly dueto the leakage in the cylinder 114′ (e.g. less than 50 mm/s). A wedgedin person thereby has sufficient time to react. In the case of a fastvertical movement of the manipulator 10 (emergency stop type B), the airinflow passage of the cylinder is switched to a reserve tank 120 filledwith compressed air. In the case of a switch-off of power, this can takeplace, e.g. automatically by means of a spring. The compressed air inthe reserve tank is sufficient to lift the weight up to the upper endstop (minimal deflection z_(B) of the actuator) and to hold it foraround 1 minute despite a leakage in the cylinder. The differencebetween a “critical deflection” z_(D) of the actuator, upon which theemergency stop of type B is carried out, and the minimal deflectionz_(B) of the actuator is, in this case, considerably larger than themaximum braking distance of the manipulator 10, so that the workpiece iseffectively raised (against the movement of the manipulator uponbraking) in case of an emergency. Given a difference (z_(B)−z_(D)) of 25mm and a braking distance of 9 mm, the workpiece 50 would be effectivelyraised by 16 mm. The raising in this case takes place at a higher speedthan the maximum speed of the manipulator 10 during braking.

To restart operations, the normal operating pressure is again applied tothe actuator, whereby the workpiece is raised; the manipulator 10 canthen raise the workpiece 50 further and optionally move back to a startposition or continue the process interrupted by the emergency stop.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open-ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for operating a manipulator and apneumatic actuator which is arranged between the manipulator and anobject to be placed on a placement surface, the method comprising:picking up the object and moving the object, by the manipulator, to afirst position in a vicinity of the placement surface on which theobject is to be placed; moving the object by the manipulator towards theplacement surface, with the pneumatic actuator being controlled suchthat an actuator force of the pneumatic actuator at least partlycompensates a weight force of the object and an adjustable minimal netactuator force acts on an end stop of the pneumatic actuator; monitoringa deflection of the pneumatic actuator and detecting a change of theactuator position; stopping the motion of the manipulator upon detectionof a change of the actuator position; releasing the object; and as asafety measure, braking the manipulator and connecting an air inflowpassage of the pneumatic actuator to a compressed air source whichcauses the pneumatic actuator to carry out a movement which at leastpartly compensates movement of the manipulator during the braking. 2.The method of claim 1, wherein the compressed air source is a reservetank.
 3. The method of claim 1, further comprising: reducing theactuator force before the release of the object, such that the netactuator force acting on the placement surface is substantiallydetermined by the weight force of the object.
 4. The method of claim 3,wherein acceleration forces which arise due to the motion of themanipulator are taken into account on the regulation of the net actuatorforce.
 5. The method of claim 1, further comprising: reducing theactuator force before the release of the object, such that the actuatorforce is substantially zero.
 6. The method of claim 1, wherein thepneumatic actuator is nearly free of static friction.
 7. The method ofclaim 6, wherein stopping the motion of the manipulator comprises:stopping the motion of the manipulator when the actuator position of thepneumatic actuator has changed by a first predefined value.
 8. Themethod of claim 1, further comprising: initiating safety measures whenthe actuator position has changed at least by a predefined second value.9. The method of claim 8, wherein initiating safety measures comprises:braking the manipulator; and connecting an air inflow passage of thepneumatic actuator to a compressed air source, so that the pneumaticactuator carries out a movement which at least partly compensates themotion of the manipulator during the braking process.
 10. The method ofclaim 8, wherein, as a safety measure, the pneumatic actuator is blockedby means of valves so that a further movement of the pneumatic actuatoris only possible due to leakage in the pneumatic actuator.
 11. Themethod of claim 1, further comprising: during picking up or placing theobject on the placement surface, detecting contact between the pneumaticactuator and the object or between the object and the placement surfaceby detection of a change of the actuator position.
 12. The method ofclaim 11, wherein the size of the object or deviations from a specifiedsize, or both, are determined from the actuator position and theposition of the manipulator on contact.
 13. The method of claim 12,further comprising: indicating when a deviation from the specified sizeof the object exceeds a predefined value.
 14. The method of claim 11,wherein a resilience or stiffness of the object or of the placementsurface is determined from the actuator position and the actuator forceon contact.
 15. The method of claim 1, wherein the picking up of theobject comprises: monitoring the actuator position and detecting contactbetween the pneumatic actuator and the object by detecting a change ofthe actuator position; and stopping the motion of the manipulator upondetection of contact.
 16. A method for placing an object using amanipulator which has a pneumatic actuator arranged between amanipulator and the object to be positioned, the method comprising:controlling the pneumatic actuator such that an actuator force of thepneumatic actuator in a vertical direction corresponds to a weight ofthe object; initiating one or more safety measures if an actuatorposition changes by at least a predefined second value; and as a safetymeasure, braking the manipulator and connecting an air inflow passage ofthe pneumatic actuator to a compressed air source which causes thepneumatic actuator to carry out a movement which at least partlycompensates movement of the manipulator during the braking.
 17. Anapparatus, comprising: a manipulator; a pneumatic actuator coupled tothe manipulator such that the pneumatic actuator is arranged between themanipulator and an object to be positioned; and a robot control which isconfigured: to control the pneumatic actuator such that an actuatorforce of the pneumatic actuator approximately compensates a weight forceof the object, to monitor the actuator position of the pneumaticactuator and initiate safety measures when the actuator position exceedsa predefined value, and to initiate, as a safety measure, a brakingprocess of the manipulator and to connect an air inflow passage of thepneumatic actuator to a compressed air source which causes the pneumaticactuator to carry out a movement which at least partly compensatesmovement of the manipulator during the braking process.
 18. Theapparatus of claim 17, wherein the manipulator has a robot arm movablein a horizontal plane and a lifting apparatus which is configured toposition the robot arm in the vertical direction.
 19. The apparatus ofclaim 17, wherein the compressed air source is a reserve tank.
 20. Theapparatus of claim 17, wherein the robot control is configured to: movethe object, by the manipulator, to a first position in a vicinity of aplacement surface on which the object is to be placed; move the objectby the manipulator towards the placement surface; detect contact withthe placement surface by detecting a change of the actuator position;stop the motion of the manipulator upon detection of contact; andrelease the object.